There are a number of pharmaceutical compositions which suffer from instability problems due to the fact that the active component is susceptible to certain types of degradation, thereby diminishing their attractiveness and, in some cases, rendering them unsuitable from a commercial standpoint. For example, several ACE inhibitor-containing compositions suffer from this drawback since certain ACE inhibitors degrade readily in pharmaceutical dosage forms. For example, quinapril, enalapril, and spirapril degrade readily in dosage form to a diketo piperazine (the internal cyclization product) and a diacid (the ester hydrolysis product). It is believed that one or more of these types of degradation including oxidation causes the discoloration in pharmaceutical compositions containing ACE inhibitors. In addition, the degradation products may result in decreased drug effectiveness in such pharmaceutical compositions. Accordingly, in view of their usefulness in treating hypertension, a number of research endeavors have been directed to overcoming the instability problem associated with pharmaceutical compositions containing ACE inhibitors, with limited success.
Various methods of improving the stability of certain ACE inhibitors have been disclosed. U.S. Pat. No. 4,743,450 discloses that certain ACE inhibitors, and in particular, quinapril and its acid addition salts can be stabilized by making solid compositions that include an alkali or alkaline earth metal carbonate, preferably magnesium carbonate, and a saccharide, specifically a sugar, such as mannitol or lactose. U.S. Pat. No. 4,793,998 discloses that certain ACE inhibitors, and in particular, quinapril and its acid addition salts can be stabilized by making solid compositions that include ascorbic acid, and optionally one or more acids selected from citric, fumaric and maleic acids. U.S. Pat. No. 4,830,853 discloses that certain ACE inhibitors, and in particular, quinapril and its acid addition salts can be stabilized by making solid compositions that include ascorbic acid or a metal or ammonium ascorbate.
Although each of the above patents represents an attempt to overcome the instability problems associated with pharmaceutical compositions containing an ACE inhibitor, there still exists a need for improving the stability of such pharmaceutical compositions, especially in the presence of moisture.